The present invention relates to a writing instrument and, more particularly, to a device for advancing and retracting a writing element in a writing instrument such as a ball-point pen.
There has been well known to those skilled in the art a device for advancing and retracting a writing element of the type in which a forward cam portion of a knocking rod cooperates with a rotary cam follower which coacts with a stationary cam formed on the inner wall surface of a sheath body. When the knocking rod is knocked or pushed forward by the thumb, the rotary cam follower engages with or disengages from inclined cam surfaces of the stationary cam so that the writing element is advanced out of or retracted into the writing instrument body.
In the case of the writing-element advancing and retracting device of the type described above, the writing element is always resiliently biased against the rotary cam follower, so that when the writing element is allowed to retract into the body with the rotary cam follower disengaged from the stationary cam, the knocking rod will be forced toward a rear opening of the body due to the resilient biasing force. Therefore, the knocking rod will be slipped rearward out of the rear opening if a suitable countermeasure is not provided to prevent this. In order to eliminate this problem, there have been proposed various structures.
For instance, in a conventional advancing and retracting device, a radially inwardly projecting engaging stop is formed on the inner wall of the body so that when the writing element is retracted, an engaging portion of the knocking rod is brought into abutting engagement with the engagement stop, thereby preventing the knocking element from slipping out of the writing instrument body. Furthermore, in another conventional advancing and retracting device, an inward flange is formed in the vicinity of the rear opening of the writing instrument body in opposing cooperative relation with projections extending radially outwardly from the outer surface of the knocking rod, whereby the knocking rod is prevented from slipping rearward out of the body.
In both cases, it is impossible in the assembling operation to insert the knocking rod into the body through its rear opening since the knocking rod has a diameter greater than the inner diameter of the inwardly directed engaging stop or flange of the body. Therefore, the knocking rod must be inserted into the body through the front opening.
The rear end portion of the knocking rod, or knocking head exposed out of the rear opening of the body is a component part to which is imparted the knocking or pushing force by the thumb of the user. It is therefore desirable to make the diameter of the knocking head as great as possible from the viewpoint of good outer appearance and smooth knocking operation. In the case of the writing element advancing and retracting device described above, the knocking head is a member separate from the knocking rod and fitted to the rear end of the knocking rod in an assembling step carried out after a step of inserting the knocking rod into the body. Therefore, the knocking head is naturally greater in diameter than the knocking rod.
The above fact raises various problems that the number of component parts of the writing instrument increases and special jigs must be used when the knocking head is attached or fitted to the rear end of the knocking rod so that productivity is low and the fabrication cost is high.
Moreover, due to the fact that the knocking head must be inserted into the body through the front opening thereof, the writing instrument body must be maintained upright with the rear opening directed upwardly, until the knocking rod has been fitted and assembled in position within the body, in order to prevent the knocking rod from slipping out of the body through the rear opening.